This invention relates to a magnetic head preferably adapted for being loaded in a recording and/or reproducing apparatus, such as, a video tape recorder (VTR) or a data storage.
As a magnetic head employed in a 8-mm VTR, a magnetic head, as shown in FIG. 1, has been proposed in which a pair of magnetic core substrates 103, 104 having gap films 101, 102 formed thereon by sputtering are abutted to each other on the gap films 101, 102 as abutment surfaces and are bonded to each other to be integrated into one unit by a fused glass 105, with the abutment surfaces forming a magnetic gap functioning as a recording/reproduction gap.
Normally, the magnetic head of such structure employs a soft magnetic oxide material, such as Mn--Zn ferrite or Ni--Zn ferrite for the magnetic core substrates 103, 104. On the other hand, the magnetic head employs an oxide non-magnetic material, such as SiO.sub.2, for the gap films 101, 102.
Meanwhile, since SiO.sub.2 is quartz having a coefficient of thermal expansion nearly equal to zero, a residual stress is generated to the ferrite in the heating and cooling processes required for glass fusion. Also, since SiO.sub.2 tends to apply a stress to the magnetic core substrates 103, 104 in sputtering formation, the stress lowers magnetic permeability and soft magnetic property of the ferrite portion on which a film of SiO.sub.2 is to be formed. As a result, efficiency of the magnetic cores is lowered.
In addition, as SiO.sub.2 is contained in the fused glass 105, erosion of the gap films 101, 102 occurs in the fusion process, causing the glass to contact the ferrite, as shown in FIG. 2. The contact of the glass with the ferrite is also caused by structural defects, that is, a large molecular structure with SiO.sub.2 as an oxide and a rough structure of the film.
The contact of the glass with the ferrite causes atomic movement on the boundary face, and subsequent elution of ferrite components into the glass causes slight magnetization of the glass. Thus, a magnetic flux between the cores is more likely to pass through the magnetized glass in the gap and a glass part 106 near a track width restriction groove. As a result, an originally required magnetic flux from the magnetic gap to a medium is weakened.
At the end of the magnetic gap, the structure of SiO.sub.2 tends to be rough and chemical potential on the surface rises, causing erosion of SiO.sub.2. As a result, the ferrite is eluted and then precipitates again in the cooling process occur. Thus, the ferrite may appear to be continuous in optical observation. In this case, efficiency of the magnetic gap is significantly lowered. Also, the elution of the ferrite causes composition deviation of the ferrite itself and hence deterioration of soft magnetism in the vicinity of the magnetic gap, resulting in deterioration of magnetic head performance.
Such phenomena tend to be promoted by an increase in load for pressing the magnetic core substrates 103, 104 to each other and also by a decrease in thickness of the gap film. For this reason, with a narrower track width and a shorter gap length for recent high-density magnetic recording, a relative stress on the narrower track is increased and the gap film is caused to be thinner, thus raising the possibility of occurrence of the above phenomena. Consequently, head output is reduced and the narrowing of the track is disturbed.
Thus, a low-temperature fusion method has been proposed by which erosion of SiO.sub.2 due to glass and the contact of the glass with ferrite are rarely caused. The low-temperature fusion includes an etching treatment using a sputtering phenomenon, called inverse sputtering, on a gap film formation surface prior to the formation of the gap films on the magnetic core substrates by sputtering. The inverse sputtering herein means an etching treatment to the surfaces of the magnetic core substrates by sputtering Ar and N ions to the magnetic core substrates instead of a target of normal sputtering.
In further detail, in the low-temperature fusion method, the gap formation surfaces of the magnetic core substrates processed with a predetermined treatment are abraded so that the arithmetical mean deviation of profile Ra prescribed by JIS B0601 is 20 to 100 .ANG.. After the gap formation surface of at least one of the magnetic core substrates is inversely sputtered to be a smooth surface, the gap film of SiO.sub.2 is formed on the gap formation surface by sputtering.
Then, the pair of magnetic core substrates are abutted to each other on the gap formation surfaces as abutment surfaces with a gap spacer between them, and are bonded to each other to be integrated into one unit by a fused glass of appropriate composition. In the low-temperature fusion, viscosity of the fused glass in fusion is prescribed. As the result of studies by the present inventors, it has been found that the viscosity of the fused glass in fusion may be 160000 (Pa.multidot.s) for sufficiently filling the fused glass into a space between the magnetic core substrates for preventing the erosion of SiO.sub.2 due to the fused glass. It has also been found that the fusion temperature may be 520.degree. C. for obtaining the above-mentioned viscosity in fusion in case where a certain fused glass exhibits viscosity in relation to temperatures as shown in FIG.3. In FIG.3, the viscosity is expressed by log.eta.. Thus, the low-temperature fusion is carried out at 520.degree. C. using the glass having the temperature-viscosity characteristics shown in FIG. 3.
As a result, a magnetic head having a magnetic gap serving as a recording/reproduction gap between abutment surfaces of magnetic core substrates is formed.
In this magnetic head, the erosion of SiO.sub.2 at the end of the magnetic gap, the contact of the glass with the ferrite, and the weakening of the leakage magnetic flux from the magnetic gap are less likely to occur than in the previously-described magnetic head, thus improving characteristics.
However, the production of the magnetic head using the low-temperature fusion causes the following inconvenience. That is, a deviation in temperature of the fused glass in fusion excessively affects quality of the magnetic head as the end product, and the allowable range of the fusion temperatures is too narrow, thus resulting in unsatisfactory productivity.
At a fusion temperature lower than the optimum temperature, the fused glass is not sufficiently filled in the space between the magnetic core substrates. On the contrary, at a higher fusion temperature, gap films 111, 112 of SiO.sub.2 formed on magnetic core substrates 113, 114 are eroded by a fused glass 115, as shown in FIG. 4.
Consequently, it is necessary to limit the fusion temperature difference to the minimum level, and to employ a fusion furnace of high temperature precision while implementing sufficient temperature control. Thus, mass production is difficult and productivity is unsatisfactory. In addition, if mass production is carried out using the low-temperature fusion, irregularity tends to be generated in degree of erosion of SiO.sub.2 among magnetic heads. Thus, mass production is difficult and productivity remains unsatisfactory.
Even though the fusion is carried out at the optimum fusion temperature in producing the magnetic head with the technique of low-temperature fusion method, the fused glass reacts with SiO.sub.2, forming a reaction layer 118 on the boundary face between the fused glass 115 and the gap films 111, 112, as shown in FIG. 5. Thus, it is impossible to perfectly prevent erosion of SiO.sub.2.
This has also been apparent from the following fact. A film of SiO.sub.2 was formed with a thickness of 850 .ANG. on a substrate formed of magnetic ferrite, and a fused glass having a thickness of 2000 .ANG. estimated to be required for producing the magnetic head with the low-temperature fusion method was formed on the film of SiO.sub.2. The resulting product was heat-treated at 530.degree. C. for one hour. The analysis of the boundary face between SiO.sub.2 and the magnetic ferrite was conducted with the Auger Electron Spectroscopy. The result is shown in FIG. 6. From this result, it can be found that SiO.sub.2 reacts with the fused glass, forming the reaction layer on the boundary face, and that erosion of SiO.sub.2 is not perfectly prevented. In FIG. 6, abscissas indicate the depth in the direction of film thickness with 0 indicating the surface of the fused glass, while ordinates indicate relative concentration.
Even though the magnetic head is produced with the low-temperature fusion method, as long as the SiO.sub.2 film is used as the gap film, with the coefficient of thermal expansion of SiO.sub.2 nearly equal to zero as described above, a stress is generated to the portion of the magnetic core substrate in which the SiO.sub.2 film is formed. Thus, there is a possibility that SiO.sub.2 is eroded.